This invention relates to image scanners for use in facsimile machines and the like. Document scanning devices transform images on paper (text or art) into a digital format suitable for processing by a computer. This data base can then be used for a host of applications ranging from facsimile (FAX) machines to copying machines.
Current scanning devices rely on optical lensing to focus the image on paper onto the image plane of a "camera tube". Modern "camera tubes" are actually solid state charge coupled devices (CCD) or photo-junction sensors that convert the image into a digital signal.
In scanning the paper image, two approaches have been taken: area array scanning and line scanning. In area array scanning, "snap shot" composite pictures are actually of the document as it passes the viewing window of the scanning optical lensing (similar to composite aerial photography). An "area" CCD device (with other electronic circuits) then digitizes each "snap shot" and electronically "glues" these images back together inside computer memory. Once this electronic photograph is created, it can be used for any application where digitized image scanning is required (facsimile FAX machines, solid state paper copiers, computer document scanner, inspection machines, etc.).
The drawback for area scanners is in their limited resolution and cost. State-of-the-art resolution for an area CCD ranges from 128.times.128 lines square to 2000.times.2000 lines square. For use on an 81/2 inches-wide documents, this represents a horizontal resolution of from 14 to 200 lines per inch. As a reference point, 200 lines per inch is still very coarse resolution for photographs, while newspaper quality pica type requires at least 70 lines per inch for proper font reproduction.
Area black and white (B/W) CCD pick-up devices are expensive and increase greatly in cost as the resolution and span coverage increase (above 81/2 inch scan width). As an example, a zero defect 128.times.128 high speed area array can cost over one thousand dollars. Other arrays with higher resolution can exceed $10,000 just for the imaging CCD! (e.g. the Kodak 4-Mpixel imager; Electronic Engineering Times, Dec. 12, 1988. p. 14) The high cost of CCD's forces many imaging applications to use hybrid solutions where mechanical scanning is combined with CCD technology. A fast full feature FAX machine with no pixel defects and capable of 200 lines per inch resolution can sell for over two thousand dollars. Even the least expensive B/W FAX machine (with slow mechanically scanned CCD carriages) will still cost over $600.00. Low cost ($300 or less), high resolution scanners simply do not exist today.
In line scanning, linear photodiode or CCD arrays scan one line at a time as the document passes the sensor strip. This technique also requires optical lensing and supporting optical hardware. Sensor strip scanner integrated circuits are also very expensive; for example, a zero defect 1024.times.1 linear array can cost over one thousand dollars (e.g. the Reticein RL1024C).
Black and white grey shade scanners are still premium devices available only to high level industry. In general, these scanners are very expensive and run just behind color scanners in high cost.
Color scanning devices are now afforded only by industry and run over $15,000. They use very expensive in-line strip CCD sensors, plus the document must be scanned three times (one for each primary color) for color encoding. A severe problem with such machines is maintaining exact alignment of the three scans so that color bleeding and loss of resolution does not occur.
Conventional optical scanners have many parts. This adds to product cost and therefore, selling price. This is one reason why low-cost scanners are currently not available. The following items highlight the major cost variables for a modern day image scanner:
a. Optics. Major cost items include a camera pickup device with scanning electronics, optical lenses, focus system, and strip aperture system.
b. Drive System. The mechanical drive system must be highly precise and requires stepper motors coupled to precision gears. Electronics to drive the stepper and precision parts are costly.
c. Illumination. A bright and flat source of illumination is required. This source must span a minimum of 9 inches and typically uses a gas discharge tube bulb powered by a high voltage power supply. In addition, green light filtering is necessary since CCD imaging devices must use this color to achieve maximum resolution.
Large document scanners for "D" size prints or drawings are not known to exist since it is quite costly to produce a 36 inch scanner (suitable for E size industrial prints) that offers 200 lines per inch resolution.